Conventionally, a master apparatus and a slave apparatus communicate with each other. The master apparatus and the slave apparatus each use a reference device such as a crystal oscillator to generate reference frequencies as accurate as possible. However, many communication devices communicate with each other in a vehicle, and the communication devices use respective reference devices to generate reference frequencies. Thus, the accuracies of the reference frequencies generated from the reference devices vary.
If a crystal oscillator is used as the reference device, the reference frequency varies with a change in the environment temperature. The frequency accuracy also degrades due to aging degradation. These characteristics affect the communication quality. To solve this problem, recent reference devices use a temperature compensated crystal oscillator (TCXO) or a circuit to compensate for aging degradation.
Because performance of vehicular systems is improving, and many sensors are mounted on the vehicular systems, the number of communication devices increases and the number of reference devices also increases. Vehicular systems are requested to operate normally even under severe environments such as high or low temperature and strong vibration, and the above-described problem become more remarkable.
For example, the use of a crystal oscillator for the reference device increases a mounting area for the circuit and the power consumption. This makes it difficult to provide a small, lightweight, and simple system.
The technology described in JP-A-2005-303632 uses the following system configuration to reduce crystal oscillators and ceramic oscillators. A master reads the main clock frequency from a slave to generate an adjustment signal that adjusts the main clock frequency of the slave to be equal to a main clock frequency of the master. The master generates transmission data based on the adjustment signal.
The slave detects an SMC adjustment signal using the transmission data from the master and adjusts the main clock frequency of the salve to be equal to the main clock frequency of the master based on the SMC adjustment signal. The master and the slave transmit and receive data to set the main clock frequencies and communication carrier frequencies. However, because the above-described method processes the main clock frequencies and the communication carrier frequencies for the master and the slave, a complicated feedback process is required. If the master communicates with multiple slaves, communication channel transmission characteristics may change or the communication may be disabled depending on the number of slaves or routing. The above-described feedback process is unavailable in these states.